The present invention relates to combined cycle electric power plants and more particularly to improved gas turbine startup controls especially useful in the operation of backup control systems for gas turbines in combined cycle electric power plants.
A gas turbine that is used for electric power generation is typically started with a diesel engine or other power assist device. When the turbine reaches the ignition speed which may be 20% of synchronous speed, lightoff occurs and the diesel continues to drive the turbine until it reaches a self-sustaining speed which may be 60% of synchronous speed. Thereafter, the turbine is accelerated to synchronous speed and synchronization is performed under automatic or operator control. In combined cycle and other electric power plant applications, it is important that startup occur rapidly without unnecessary foreshortening of turbine life and that the plant availability be highly rated because of the reliability with which turbine startup can be expected to occur.
Various types of startup control have been employed for electric power gas turbines with various types of hardware technologies including relay-pneumatic, analog electropneumatic, and digital/analog electropneumatic. With respect to the different types of controls, it has been common to start a gas turbine with a temperature control loop, i.e. the turbine is caused to accelerate to synchronous speed at a rate allowed by an exhaust temperature limit and in some cases with a surge limit override. Faster startups are permitted on cold days with temperature limit startup control.
In a digital computer/analog hybrid control set forth in the previously referenced patent application Ser. No. 082,470, fixed time gas turbine startup is disclosed in which a speed reference is digitally generated and applied to a speed control loop so that startup occurs in a fixed time even though day-to-day ambient temperature variations occur. Another electropneumatic startup control is disclosed in Ser. No. 371,635 assigned to the present assignee.
In U.S. Loft Pat. No. 3,520,133, an automatic startup control is disclosed in which analog electronic and pneumatic hardware is employed and in which a feedforward startup control is employed with temperature and acceleration limit control. In the Loft patent, the startup control is a part of the primary control system.
In much of the prior art, little consideration is given to backup startup controls for the primary and usually automatic control loop. Often this is because of the presence of multiple control loops in the automatic controller itself which serve to some degree as backup for each other especially when the control loops are embodied in relatively independent hardware paths as in an analog electropneumatic control. In digital computer/analog hybrid controls, the need for a backup control for startup and other control purposes can be greater because the whole automatic control may be put out of service with the failure of the computer. Thus, in copending and coassigned patent application Ser. No. 476,182 entitled "Local Maintenance Controller For Gas Turbine Power Plants Having A Primary Control System" filed by R. A. Yannone et al on June 4, 1974, there is a disclosed a backup control for a turbine control system which employs a general purpose digital computer.
In various power plant applications, it is generally desirable to provide a simple yet reliable control for gas turbine startup operation. More particularly, in various electric power plants and especially in combined cycle power plants, it is desirable to provide a plant control system with a simple yet reliable gas turbine startup control which can serve as a backup startup control for the automatic controller in the event of a failure of the automatic controller or which can serve to provide plant startup or turbine startup upon operator selection of the manual mode.
The description of prior art herein is made on good faith and no representation is made that any prior art considered is the best pertaining prior art nor that the interpretation placed on it is unrebuttable.